Sound recording apparatus



Dec. 21, 1937. BAKER 2,102,776

SOUND RECORDING APPARATUS Filed May 28, 1936 2 Shets-Sheet 2 wumwm,

3nventor wwzd 0. flake];

attorney Patented Dec. 21, 1937 I PATENT OFFICE SOUND RECORDING APPARATUS I Judd 0. Baker, Westinont, N. J a ss ignor to Radio Corporation of America, a corporation of Dela ware 7 Application May 28, 1936, Serial No. 82,375 6 Claims. 01. 179-1003) The present invention relates to sound recording apparatus and more particularly to sound recording apparatusof the type adapted to record sound photographically upon film.

The recording of sound on film by what is known as the variable area. method has been known for a number of years and is described, for example, in Fritts Patent No. 1,203,190. In the reproduction of the earlier variable area sound records, as well as variable density sound rec- I ords, there is produced what is known as ground noise due to imperfections in the film and. to

noise produced in the photo cell, and in these earlier films the volume of this noise was constant as the average light transmission was constant and accordingly, it was very objection,- able at low sound volumes. It was accordingly proposed as, for example, in Robinson Patent No. 1,854,159 to reduce the transparent area of the positive film at low sound amplitudes thereby decreasing the ground noise as the volume of sound was decreased and correspondingly improving the quality of the sound at low amplitudes.

It was proposed in McDowell Patent No.'l,855,- 197 to use a shutter in-the sound recorder to cover, and thereby render transparent, the unused; portion of the sound negative which in turn rendered the corresponding portion of the positive film opaque in order to reduce ground noise. My invention is an improvement on the said McDowell apparatus.

' One object of my invention is to provide a sound record which is symmetrically arranged in relation to the sound trackarea.

'3 Another object of my invention is to provide an improved groundnoise reduction shutter.

" Another object of the invention is to provide an attachment for 'conventionalrecording apparatus which will serve to convert it to produce my improved record.

Other and incidental objects of my invention will be apparent to those skilled in the art from 'a'r'eading of'the following specification and. an inspection of the accompanying drawings in WhiQh-r- Fig. 1 is a schematic representation of a sound 7 recording apparatus built in accordance with my invention, 7 Fig. 2 is a horizontal section through my improved optical system and shutter arrangement,

Fig. 3 is a vertical section thereof,

Fig. 4 is an end view looking from the right hand side of Figs. 2. and 3, 1

;. Fig. 5 shows details of the construction,

Fig. 6 shows a positive print of my improved sound record,

Fig. '7 is an enlarged view showing the relation of the ground noise reduction shutters to the recording aperture, 7 V

Fig. 8 is a schematic diagram of the relation of the shutters to the recording aperture, 7

Fig. 9 shows the relation of the image of the recording aperture to the slit with the shutters in closed position, and

.which may be constructed in accordance with the said McDowell patent. This amplifier 6 amplifies the current fed to it, then rectifies the amplied current and passes the rectified current through a low-pass filter, thus providing a variable direct current output, the instantaneous value of which corresponds to the average value of the sound input to the galvanometer. Such a current is commonly termed an envelope current, as it corresponds to the envelope of the sound waves. 7

In the recorder, light from an exciter lamp 1 is passed through a condenser lens 8 and an aper- 5 ture plate 9 to the lenses l0 and II. These lenses in and I I concentrate the light on the galvanometer mirror 5 andalso serve to focus an image of .the aperture l2 in the plate 9 on the slit plate 7 l3. This slit plate is provided with a narrow slit 40 I4 which serves to select the desired portion of the image I2, which is then focused on the film I5 by the lenses I6 and I1. As shown in the drawings, the aperture I2 is in the form of an elongated rectangle with two triangular exten- 45 sions on one side and a smaller rectangular extension on the other side. When this aperture is imaged on the plate l3, as indicated at I 2', the triangular portions fall on the slit l4 and produce two areas of illumination of the slit., 5 When the galvanometer mirror 5 is oscillated about a horizontal axis, these triangular areas move up and down across the slit 14, thereby causing the'slit to intercept portions thereof of different lengths and correspondingly. producing two separated sound records, each corresponding to the sound being recorded. Light from the rectangular extension in the aperture 52 is deflected by the prism l8 and is again reflected by the prism l9 onto the screen 23, where the movement of this portion of the spot of light permits the operator of the apparatus to see the manner in which the galvanometer is operated.

Current from the ground noise reduction amplifier 6 is passed to a magnetic means 2!, which may be of any desired type, where it is causedto deflect a movable member 22 in accordance with the amplitude of the envelope current. .As the member 22 is deflected it pulls upon a flexible resilient member 23, thereby causing the shutters 24 and 25 to move about the pivots 26 and 21 against the tension of the spring 28. It will be apparent that the amplifier unit 6 may be so connected as to cause an increase in the en velope current when there is a decrease in the output of the amplifier 2, thereby causing the shutters 24 and 25 to be moved toward each other and masking off a corresponding portion of the aperture l2. Alternatively, the shutters 24 and 25 may be mechanically adjusted toremain in their most nearly closed position and when there is an increase in output from the microphone l, the output from the amplifier and rectifier 6 would then be so connected as to cause the shutters to be opened by the increasing current.

I prefer, however, to have the shutters mechanically adjusted so that when their circuit is open they are in the position shown in Fig. 1; to adjust the amplifier and rectifier 6 so that when the circuit is closed, the shutters 24 and 25 are pulled to their closed position when there is no input; and when modulation is applied to the amplifier 2, and the rectifying amplifier 6 to cause the current through the magnetic means 2i to decrease, thereby permitting the shutters to open in accordance with the amplitude of the sound to be recorded. It will be apparent that the selection of any one of these three methods can be accomplished by merely a reversal of connections either between amplifier 2 and the rectifying amplifier 6, between amplifier 6 and the magnetic means 2!, or of magnetic means 2! to the arm 22. Similarly, if it is desired to cause a decrease in the output of the amplifier 6 or an increase in the output of the amplifier 2, it is necessary that the output of the amplifier 2 increase the negative grid bias in the rectifying amplifier 6, while to produce the opposite result, the connections are so made as to cause a decrease in the grid bias.

Referring now to Figs. 7 to 10, the aperture plate 9 is located quite close to the shutters 24 and 25 and when the shutters are in the closed position, i. e. when there is nosound being recorded, each covers approximately one-fourth of the aperture l 2 as shown in Figs. 7 and 8. When the image of this covered aperture is projected on the slit plate l3, which is shown greatly enlarged in Figs. 9 and 10, it presents the appearance shown in Fig. 9, where only narrow tips of the image l2 coincide with the slit, thereby producing only a very narrow record such as indicated at 3B in Fig. 6. When, however, modulation is applied to the amplifier 2, the shutters 24 and 25 are withdrawn toward the position shown in Fig. 1, a distance corresponding to the amplitude of the sound to be recorded, and when the shutters are completely withdrawn, for recording maximum amplitude, the image !2' on the slit'plate l3 presents the appearance shown in Fig. 10. It will be apparent that when the image l2 in Fig. 10 is moved vertically by oscillation of the mirror .2 it will swing between two extreme positions, in the lowermost position only the tops of the triangles of the image coinciding with the slit and producing a very narrow record portion, as indicated at 3!, and at the uppermost position substantially the entire slit being filled with light, thereby producing the peaks indicated at 32 in Fig. 6. As a consequence of the described movement of the shutter in accordance with the modulation, a negative record will be producedfrom which a positive may be printed having the. appearance shown in Fig. 6, from which it will be apparent that the record has an outer envelope corresponding with the envelope of the voice currents and an inner outline corresponding with the sounds themselves, the outer envelope being produced by the shutter and the inner outline being produced by the galvanometerj It will be apparent that each of the halves of thisrecord corresponds with the McDowell record but that two records are symmetrically ar ranged with relation to the sound track, thereby preventing the cutting off of peaks at high amplitudes if the reproducing aperture is slightly out of alignment by preventing the cutting out of the sound under similar circumstances at extremely low amplitudes which might follow with such sound records, as for example that of Hanna Patent No. 1,888,724.

The specific shutter mechanism is described in greater detail and claimed in the concurrently filed applications of myself and Cecil N. Batsel-- Docket i613 and the application of Harold J. Hasbrcuck-Docket 4679. It includes a base member 46 which may be the base of the recorder. Upon this base member is mounted a support member 4| on which the portion of the optical system including the condenser 8, the lens Ill, and the intervening shutter mechanism is mounted. On this is mounted an appropriate carriage 42. As shown in Fig. 4, this carriage is mounted for lateral movement on the member 41, being shifted by the screw 43 provided with the jam nut 44 and being held firmly in position by the compression spring 45 which produces a pressure opposing that exerted by the screw 43. This carriage 42 includes a cylindrical upper portion 46 in which the barrel 4'! of the optical system is fitted.

In order to position the optical system with the aperture 12 accurately parallel to the slit I4, the optical barrel is 'slitted at .48 and into this slit is fitted an eccentric pin 49 on a screw 50. It will be apparent that rotation of the screw 50 will cause the pin 49 to move laterally of the member 46 and to thereby rotate the barrel together with the aperture plate 9. The barrel may not only be rotated but may be slid longitudinally within the limits prescribed by the slots 48 .and it can then be clamped in place by the screw 5 I which passes through the slit ring 52 and which forms a portion of the member 46.

As shown in Fig. 2, the shutters 24 and 25 are pivoted on the knife edges 26 and 21, being held thereagainst by the spring 28. These shutters 24 and 25 are adjustably secured on the arms 53 and 54 by the screws 55 and 56 in order that they may be adjusted to the proper distance from the aperture plate 9. It will be apparent that although the ends of the shutter blades must clear the aperture plate 9, they must, at the same time, be so close thereto that a properly focused image of theedges of the shutters will be formed longitudinally in the guideway, thereby shifting the pivots of the shutters and. correspondingly shifting the opposite'ends of the shutter blades in the opposite direction to the movement of the pivots.

As before described in connection with Fig. 1,

a resilient member 23 is connected from the arm 22 to the arms supporting the shutters and when this arm is actuated by the magnets the shutters are pulled closer together or moved farther apart, as the case may be.

vIn the preferred form of the invention, as shown in Figs.2 to 5, the arm 23 is provided with a rectangular armature 63 as shown in dotted lines in Fig. 3, and moves in a plane passing through the axis of the lenses between the poles of the pairs of magnets 64 and 65. The coils of the several magnets 64 and '65 are connected to the ground noise reduction amplifier 6 and in such a manner that the pairs of poles longitudinally of the device are of opposite polarity, thereby causing movement of the arm 23 in accordance with the energization of the magnets and.v

correspondingly moving the ends of the shutters 24 and 25.

Having now described myinvention, I claim:

1; Apparatus of the class described comprising means for forming a, spot of light increasing in width from the middle to both ends, means for selecting a narrow line of light from said spot, meansfor vibrating said spot across said selecting means in accordance with sound waves to be recorded, and means for masking the ends of said spot in accordance with the volume of the sound to be recorded.

2. Apparatus of the class described comprising means for forming a spot of light increasing in, width fromthe middle to both ends, means for selecting a narrow line of light from said spot, means for vibrating said spot across said selecting means in accordance with sound waves to be recorded, and means for masking the ends of said spot in inverse accordance with the volume of 'the sound to be recorded.

3. Apparatus of the class described comprising means for forming a spot of light increasing in Width from the middle to both ends, a slitted member for selecting a narrow line of light from said-spot, a galvanometer for vibrating said spot across said slitted member in accordance with sound waves to be recorded, and a pair of shutters for masking the ends of said spot in accordance with the volume of the sound to be recorded.

4. In sound recording apparatus, an optical unit comprising acondenser lens, a mask having an aperture with one side W-shaped, and a pair of shutters adapted to obscure the ends of said aperture in accordance with the volume of sound.

5. In sound recording apparatus, an optical unit comprising a condenser lens, a mask having an aperture with one side W-shaped, a pair of shutters adapted to obscure the, ends of said aperture in accordance with the volume of sound, and magnetic means for actuating said shutters.

6. In sound' recording apparatus, an optical unit comprising a condenser lens, a mask having an aperture with one side W-shaped, a pair of shutters adapted to obscure the ends of said aperture in accordance with the volume of sound, means for actuating said shutters, and a lens for imaging said aperture on a slit.

JU DD 0. BAKER. 

